Know your lenses

A lens focuses light onto the sensor of a digital camera and is critical to photography. To help you make an informed choice when buying or mounting an optic, we explain some of the most important features of lenses.

Camera lenses explained: Focal length

Focal length is the distance in millimetres from the imaging surface (film or sensor) and the optical centre of the lens when it is focused at infinity.

As lenses are made up of multiple elements arranged in groups it isn't always immediately apparent where the optical centre is, but, with a few exceptions, longer focal length optics are usually longer than shorter ones.

Camera lenses explained: Angle of view

With full-frame cameras - those that have a sensor that is the same size as a 35mm film frame - lenses longer than 50mm are considered telephoto optics because they have an angle of view that is smaller than our eyes.

This means that a distant object will appear larger in the frame than we see it and we often refer to telephoto lenses as getting you 'close' to the subject.

Lenses with a focal length shorter than 50mm are generally regarded as wide-angle optics because they have a wider angle of view than our eyes.

As you've probably already deduced, 50mm lenses have an angle of view that is roughly the same as our eyes.

But what is angle of view? Imagine drawing two lines from a lens's optical centre to the outer opposite edges of the scene visible through the lens, the angle between these lines is the angle of view.

Angle of view can be measured vertically, horizontally or diagonally (this gives an increasing measurement in landscape orientation). Canon's 50mm f/1.8 lens, for example, has an angle of view of 27, 40 and 46 degrees (vertical, horizontal and diagonal).

Meanwhile the angle of view of the Canon 100mm f/2 is 14, 20 and 24 degrees. These figures can usually be found in a lens' specification list to indicate how much of a scene is visible through it.

Camera lenses explained: Prime and zoom lenses

Prime lenses are lenses that have a fixed focal length, whereas zoom lenses allow you to change between focal lengths without changing lens.

Zoom lenses are extremely convenient and their quality has improved enormously over the years, but they still have to make some compromises in order to enable the focal length shift.

This means that a series of comparable prime lenses will usually produce better quality images than a zoom lens. Broadly speaking, the wider the zoom range, the greater the degree of compromise that has to be made.

Camera lenses explained: Focal length multiplier

As mentioned earlier, a prime lens's focal length is fixed. However, if you take a 50mm lens and put it on an APS-C format camera you will see less of a scene than you will if you put the same optic on a full-frame camera.

The smaller sensor size of the APS-C format camera fills a smaller part of the lens' imaging circle and effectively produces a cropped version of the full-frame image. As a result it makes the lens appear to have a longer focal length than it actually has.

This effect is often described as focal length magnification and cameras with smaller than full-frame sensors are said to have a focal length magnification factor or crop factor.

Canon APS-C format SLRs like the EOS 7D Mark II or EOS 700D etc, have a focal length magnification factor of 1.6. This means that a 50mm lens will capture an image that looks the same as one captured by an 80mm (50 x 1.6) on a full-frame camera.

Micro Four Thirds cameras have a magnification factor of 2, so a 50mm lens produces images that look like those shot using a 100mm lens on a full-frame camera.

ABOUT THE AUTHOR

Head of Testing, Cameras

Angela (Twitter, Google+, website) is head of testing for Future's photography portfolio, writing and overseeing reviews of photographic equipment for Digital Camera, Photography Week, PhotoPlus, NPhoto and Practical Photoshop as well as TechRadar's cameras channel. Angela has a degree in photography and multimedia and prior to joining Future in October 2010 was Amateur Photographer magazine's technical editor.